1. Field of the Invention
The invention relates to cathode ray tube displays and more particularly to cathode ray displays of the phosphor penetration type employing rapidly selectable high anode or acceleration voltages for providing improved color displays.
2. Description of the Prior Art
Penetration phosphor cathode ray tubes of several types are known in the art and are attractive for use in specialized displays wherein their bright color producing capabilities permit the presentation of complex data in semi-pictorial forms quickly interpretable by the viewer. They are additionally attractive for use in such information displays, including displays for air traffic control, navigation, radar, and the like systems, because they provide images with high resolution and good contrast, even in the central portion of the viewing screen.
The conventional penetration phosphor cathode ray tube in its most prevalent form has a viewing screen which uses controlled penetration of electrons into a series of phosphor layers for producing therein light of a corresponding series of colors. For example, the screen may include two or more different phosphor layers, each very thin, uniform, and separated by a transparent dielectric layer. When the electron beam is driven by a relatively low voltage, the energy of the electrons is not sufficient to penetrate the dielectric barrier layer; only the phosphor of the first layer is excited and only its color is produced. At higher electron beam voltages and correspondingly higher electron energies, the phosphors of both of the first and second layers are excited, the intensity of the color contribution by the second layer phosphor increasing as the electron beam energy is increased.
Full use may be made of the penetration phosphor characteristics in providing a variable color display while using only one electron beam simply by controlling the voltage on the beam acceleration electrode adjacent the focusing electrode. With red and green emitting phosphors, successive changes in the beam acceleration voltage generate distinct colors; for example, red, orange, yellow, or green may thus be generated. Only one suitably controlled electron gun is required and the apertured masks of prior art color television tubes are eliminated along with other features undesirable for use in fast high resolution information displays. A further understanding of the construction and operation of penetration phosphor displays is found in the following Sperry Corporation U.S. patents: S. F. Ignasiak, U.S. Pat. No. 3,939,377 for "Penetration Phosphors and Display Devices", issued Feb. 17, 1976, C. D. Lustig, J. B. Thaxter, U.S. Pat. No. 3,946,267 for "Plural Filter System Cooperating With Cathode Ray Display with Lanthanum Host Phosphor Emissive in Two Colors", issued Mar. 23, 1976, and S. F. Ignasiak, U.S. Pat. No. 4,071,640 for "Penetration Phosphors for Display Devices", issued Jan. 31, 1978.
Various techniques are found in the prior art for selectively switching the color of successive cathode ray traces on such phosphor screens from one color to another. Since such operations necessarily involve the difficult problem of switching of high anode or acceleration voltages, great care must be taken not to compound the already difficult situation by making non-constructive design choices. For example, an anode power supply which appears on its face to be simple, uses a series regulator triode in the anode voltage line and a second triode allowing the discharge of the effective capacitance of the cathode ray anode or accelerating electrode to ground. The effective capacitance to be discharged may be as high as 500 picofarads for a shielded cathode ray tube, so that considerable power must be dissipated.
Where much shorter switching times are desired and much lower power dissipation, as is often the case in airborne displays, switching power supplies have been proposed that consist of a relatively slow switching shunt triode connected in parallel with a fast switching device employing two sequentially operating pulse transformers with controlling drive circuits permitting anode voltage rise and fall times to be considerably reduced. However, even the latter arrangement lacks suitability since the two pulse transformers are large and heavy. Furthermore, the prior art supply does not generally lend itself to construction in a compact, efficient, stable, and trouble-free form.